Rotary electric machines

ABSTRACT

A single-phase alternating current capacitor-start electric motor with a stator winding wound as two components spaced 90 + OR - N* electrically, where N is a small angle but never zero. Both components are energised for running. For starting, one component is energised serially with a capacitor to provide starting torque. Phase reversal of one component reverses the direction of rotation.

United States Patent Fong et al.

ROTARY ELECTRIC MACHINES Inventors: William Fong, Westbury-on-Trym,

Bristol; Gordon Hindle Rawcliffe, Bristol, both of England Assignee:National Research Development Corporation, London, England Filed: Feb.18, 1972 Appl. No.: 227,548

Foreign Application Priority Data March 12, 1971 Great Britain..06698/71 U.S. Cl 310/185, 310/198, 318/224,

318/225 Int. Cl. H02k 1/14 Field of Search 318/224, 225;

[ 1 Sept. 25, 1973 [56] References Cited UNITED STATES PATENTS 3,440,5104/1969 Canadelli 318/224 R Primary ExaminerJ. D. Miller AssistantExaminer-H. Huberfeld AttorneyLarson et al.

[ 5 7 ABSTRACT A single-phase alternating current capacitor-startelectric motor with a stator winding wound as two components spaced 90iNelectrically, where N is a small angle but never zero. Both componentsare energised for running. For starting, one component is energisedserially with a capacitor to provide starting torque.

Phase reversal of one component reverses the direction of rotation.

7 Claims, 5 Drawing Figures SECTION 1 5 RUN 5mm 1 SECTION 2 ROTARYELECTRIC MACHINES This invention relates to rotary electric machines,particularly to single-phase capacitor-start electric motors.

The conventional single-phase capacitor start electric motor has twostator windings, the main winding and the auxiliary winding, both ofgraded concentric type and one having its axis displaced 90 electricallyrelatively to the other. Only the main winding is energised duringrunning. The auxiliary winding is energised to provide a starting torqueonly. Commonly, all the stator coils are graded according to sine laws,giving a winding factor of about 0.78.

The object of the present invention is to provide an improvedcapacitor-start motor in which the whole of the stator winding isenergised during running and no winding part is provided solely toprovide starting torque.

Accordingly, a single-phase, alternating current, capacitor-startelectric motor comprises a stator winding which is energised during therunning condition of the motor, said stator winding being wound as firstand second winding components said components being electrically spacedat an angle of 901 N where N is a small angle but never zero, and hasswitch means for energising both components during running and for'energising the first components as main winding and the secondcomponent serially with a capacitor to provide starting torque, duringstarting.

In order that the invention may be readily carried into practice, theprior art and two embodiments of the invention will now be described, byway of example, with reference to the accompanying drawings, in which:

FIG. 1 is a winding diagram representing a standardsinusoidally-distributed 2-pole concentrically wound stator winding in al2-slot frame for a known singlephase capacitor-start motor;

FIG. 2 shows similarly a stator winding arrangement in a l2-slot framefor a 2-pole capacitor start motor according to the invention;

FIG. 3 is a circuit diagram showing switching means for starting andrunning the motor of FIG. 2;

FIG. 4 shows the switching means for an alternative stator windingarrangement; and

FIG. 5 shows a series of curves indicating comparative torqueperformance of conventional single-phase capacitorstart motors and ofmotors according to the invention.

In the winding diagrams of FIGS. 1 and 2, the row of numbers at the headof the diagram represent the slot numbers of a l2-slot frame of a 2-polemotor. Equally they may represent 12 successive slots of a 2 P-polemotor wound on a 12 P-slot frame. The full lines represent the coils ofeach concentric winding and the terms below represent the turnsdistribution for each coil.

A conventional capacitor-start motor has' a winding as shown in FIG. I.The auxiliary winding normally has fewer series turns than the mainwinding and. is wound with wire of much smaller size, although it ispossible to use more turns and very thin wire. The ratios (t /t and(A,/A,,,) are respectively normally of the orders 0.6 and 0.4, orlarger, where:

r is the turns-number in series A is the conductor cross-section and thesuffices a and m refer respectively to the auxiliary and main windings.

The choice of (t /r has a great influence on the acceleratingcharacteristic for any given value of starting capacitor. For a smallvalue of (t /r the starting torque is low, but the motor is capable ofproducing high torque at higher speeds. For a large value of (r lr onthe other hand, the torque is high at starting, but drops rapidly as themotor gains speed.

The auxiliary winding is connected to the supply for part of the run-upperiod, and is then disconnected by the action of a centrifugal switch.From the view points both of copper utilisation and of labour cost inmanufacture, the auxiliary winding is undesirable. Also, burn-out of theauxiliary winding is common, as a result of stalling or too frequentstarting, because of the very high current-density in this winding.

The total copper in use in the running connection, compared with thetotal copper contained in the machine is 13,11 sin t A, sin 15 r j-1,,sin 45 t /i, sin 45 and t A sin 15 t A, sin 75 Taking the ratio (l A /t/l as 0.24, the loadings of slots 4, 5 and 6 are theoreticallyproportional to:

On average, therefore, about 0.776 of the total available slot-space isused. In practice, the slot-sizes may be graded, and some of themarginal space can then be used for fixing the core, etc., but the aboveequations necessarily represent some inherent waste of space, whether ornot uniform slotting is adopted.

In fact, the effective utilisation of the space available in the motor,when it is running, is:

Slot-filling factor Proportion used for running winding (0.776) X(0.806) X (0.78) =0.49.

X (Winding factor) The effective utilisation factor of the copper in themachine is 0.63, or less, as shown above.

These are the standards by which a winding according to the presentinvention must be judged. It is emphasised that the numerical valuesassumed above are on the favourable side for the conventional motor. Inpractice, the conventional motor is usually less satisfactory than theassumed figures indicate.

A stator winding for a capacitor-start motor according to the presentinvention is shown in FIG. 2. This motor is wound in a manner similar toa conventional motor, except that the main winding is grouped so as togive two winding sections, shown as Section 1 and Section 2, displacedby 75 electrical from each other. Winding section 1 is composed of twoidentical groups of concentric coils; the inner and outer coils of eachgroup having x and Y/2 turns, respectively. Winding section 2 has twocoils of 1 turns each. All the coilsof each section are normallyconnection in series.

The Sections 1 and 2 are connected in series to form the runningwinding. Thus, there is no part of the winding which is not used innormal operation.

The switching means to change from the Start condition to the Runcondition are shown in FIG. 3. FIG. 3 shows the circuit arrangementwhen, as stated above, Sections 1 and 2 are displaced by 75 electrical.

In an alternative arrangement, Sections 1 and 2 are displaced by 105electrical and the switching means for this variant are shown in FIG. 4.

In FIGS. 3 and 4, like elements are indicated by the same referencenumbers. Thus, Section 1 and Section 2 of the winding of FIG. 2 areindicated at 1 and 2, respectively. A starting capacitor 3 ispermanently connected to one end of winding section 1 and the junctionis connected to terminal 5 of a pair of supply terminals 4, 5. Theswitching means 6 comprises ganged switches, one having a contact 7which is connected alternatively to contacts 8 and 9, the other having acontact which is alternatively open circuit and connected to a contact11, respectively.

Contact 8 is connected to contact 10 and to one end of winding section2. The free terminal of capacitor 3 is connected to terminal 11.

In the arrangement of FIG. 3, the other end of winding section 1 isconnected to terminal 7 and the other end of winding section 2 isconnected to terminal 9. Supply terminal 4 is connected to switchterminal 9.

In the arrangement of FIG. 4, the other end of winding section 1 isconnected to terminal 9 together with the other end of winding section2. Supply terminal 4 is connected to switch terminal 7.

For the arrangements of both FIG. 3 and FIG. 4, for the startingcondition, terminals 7 and 9 are connected together and terminals 10 and11 are connected together. For the running condition, terminals 7 and 8are connected together and the capacitor 3 is disconnected by openingterminals 10 and 11.

Considering, again, the winding of FIG. 2, for uniform current-loadingin the slots, in normal operation, x y z 1 (say); and the total actualnumber of turns in the machine thus is (y 2x 2:) 5. It can then be shownthat the total effective number of turns in the whole winding, in normaloperation, is givenby: V 3 1) (1.366) 3.73; and the operative windingfactor is thus (3.73/5) 0.746.

Five slots out of six are thus fully wound, and every sixth slot isunused. The slot-filling factor is therefore 0.833, and the proportionof the winding used in normal running is 1.00.

The effective utilisation of the space available in the motor, when itis running, is:

(Slot-filling) (Proportion used for) (Winding X X factor running windingfactor 0.s 33) x (1.00) X (0.74s =0.62.

The effective utilisation of the copper is 0.746.

These figures correspond to the figures for the conventional winding ofFIG. 1, (0.49 and 0.63) calculated on the same basis.

The space utilisation is greater in the ratio (62/49) 1.26 and thecopper utilisation in the ratio (746/630) 1.18. On any basis, there is again of about 20 percent in utilisation. Manufacture is simpler and thereliability of the starting winding is greater. This is a substantialgroup of advantages.

The conductor distribution over the pole-pitch (six slots)isx:x:x:x:x:0.

For quantity production, one slot in six need not be punched. This couldprovide some space for core-fixing etc. if required.

The 2-pole example is chosen only for simplicity, but it can readily bedoubled, trebled etc. for 4-pole, 6-pole machines etc.

In an alternative form of the winding, the second section of the windingof FIG. 2 is placed in slots 5 and 6 instead of4 and 5. The angle ofelectrical displacement between winding section 1 and winding section 2is thereby changed from to 105, and the torque characteristic isimproved, as will be seen from the description given later withreference to FIG. 5.

The switching arrangement for this embodiment has been described withreference to FIG. 4.

A motor according to the present invention thus has two torquecharacteristics: one for each direction of rotation. The twocharacteristics can be interchanged by change of coil-position; or byreversal of the interconnection between the two sections of the winding,as shown in FIGS. 3 and 4. The latter reversal can, of course, be veryreadily carried out on an existing motor: the choice of coil-positionhas to be made in manufacture.

A particular design position of the second winding section gives oneparticular direction of rotation for one direction of theinterconnection between the two sections, and the other direction ofrotation for the opposite direction of interconnection. For the otherdesign position, each of these directions of rotation is reversed.

The basic reason for there being two alternative torque characteristicsin the starting regime is that there is mutual induction between the twosections of the winding, since they are not at electrical spacing. Thevalue of mutual induction will obviously be affected by the relativepositions of the sections and by their relative direction ofinterconnection. The running torque characteristic is, of course, alwaysthe same.

FIG. 5 shows the torque/speed and capacitorvoltage/speed curves of aconventional capacitor-start motor and one wound with a new type ofwinding in an identical frame; the same starting capacitor being usedfor both machines.

The main winding (only) of the conventional motor has the same number ofeffective series turns as the two sections of the new winding in series.For the same supply voltage, the torque/speed curves of the conventionalmotor (using only the main winding) and the new motor (using the wholewinding in the running connection) are therefore identical and are asgiven in FIG. 5, Curve a. Neither of these motors have, of course, anyinherent starting torque in this connection.

FIG. 5, Curve b shows the torque-speed curve of the conventional motorwith both main and auxiliary windings in circuit; the value of thestarting capacitor being chosen to give a ratio of 3:1 between thestarting and full-load torques.

Curves c and d respectively show the torque/speed relationship of thenew motor, with 75 and with I05 displacement, between the two windingsections respectively, using the same value of starting capacitor as forthe conventional motor. Curve 0 was obtained for the condition when theaxes of winding sections 1 and 2 are displaced 75 as shown in FIG. 2,and Curve d was taken with winding section 2 reversed with respect towinding section 1. This means that the spacing between the two windingsections was changed to (180 75) 105. As is shown, the torquecharacteristic is thereby substantially improved. In addition, thedirection of rotation is reversed.

Even the less favourable torque characteristic, Curve c, is appreciablysuperior to the characteristics of the conventional machine, Curve b.Curve d is better by an order of magnitude.

For extra high torque operation, in one direction the design would bebased on 105 spacing. For reversing service, it wound be necessary todecide whether the torque duty in one direction was less arduous thanfor the other direction.

The voltages on the capacitor, as the speed rises, are shown in FIG. 5,by Curves e,fand g. For 75 displacement between winding sections, Curvee, it is at all points lower than for a conventional motor Curve f. For105.displacement, Curve g, the voltage rises about to percent; and, inprinciple, this is not desirable. In practice, it is unlikely that anycapacitor used with a conventional motor will be so closely rated thatthis modest increase of voltage will not be acceptable.

The only disadvantage, for improved torque characteristics and moreeconomic use of both space and copper, is that the starting switch isslightly more complicated than for a conventional motor, as shown inFIGS. 3 and 4.

The winding described with reference to FIG. 2 is purely by way ofexample. The general principle is that it is possible to divide asingle-phase winding into two component sections; and to switch them sothat, while starting, one section of this winding acts as the mainwinding, and the other section as the starting winding.

When the machine is nearly up to speed, the two sections are combined toform the operating winding. A variety of embodiments of this newprinciple are practicable, but it is believed that the one described isone of the simpliest. Different slot-numbers, different coilgroupingsfor the two winding sections, and different turns-numbers for theindividual coils will give a variety of performance characteristics.

We claim:

ll. A single-phase, alternating current, capacitor-start electric motorcomprising a stator winding which is energised during the runningcondition of the motor, said stator winding being wound as first andsecond winding components said components being electrically spaced atan angle of 903: N degrees, where N is a small angle but never zero, andhaving switch means for energising both components during running andfor energising the first component as main winding and the secondcomponent serially with a capacitor to provide starting torque, duringstarting.

2. An electric motor as claimed in claim 1, for P poles, where P is aneven number in which the first winding component comprises P coils offull pitch, conductors of adjacent such coils sharing a common slot,together with P concentric coils of less than full pitch and the secondwinding component comprises P coils of less than full pitch.

3. An electric motor as claimed in claim 2, wound on a 6? slot frame, ina sequence of 12 slots whereof said full pitch coils extend from slot 2to slot 8, the further coils of the first winding component extend fromslot 3 to slot 7, the coils of the second winding component extend fromslot 5 to slot 10, and slot ill to slot 4, and slots 6 and 12 areunoccupied.

4. An electric motor as claimed in claim 2, wound on a 6? slot frame, ina sequence of twelve slots whereof said full pitch coils extend fromslot 2 to slot 8, the further coils of the first winding componentextend from slot 3 to slot 7, the coils of the second winding componentextend from slot 6 to slot 111, and slot 12 to slot 5, and slots 4 and10 are unoccupied.

5. An electric motor as claimed in claim 4, in the frame of which every6th slot is not punched.

6. An electric motor as claimed in claim 1, in which the first andsecond winding components are electrically spaced at an angle of 15.

7. An electric motor as claimed in claim 2, in which the full pitchcoils are wound with one-half the number of turns of each of the coilsof less than full pitch.

It t 111 t

1. A single-phase, alternating current, capacitor-start electric motorcomprising a stator winding which is energised during the runningcondition of the motor, said stator winding being wound as first andsecond winding components said components being electrically spaced atan angle of 90 + OR - N degrees, where N is a small angle but neverzero, and having switch means for energising both components duringrunning and for energising the first component as main winding and thesecond component serially with a capacitor to provide starting torque,during starting.
 2. An electric motor as claimed in claim 1, for Ppoles, where P is an even number in which the first winding componentcomprises P coils of full pitch, conductors of adjacent such coilssharing a common slot, together with P concentric coils of less thanfull pitch and the second winding component comprises P coils of lessthan full pitch.
 3. An electric motor as claimed in claim 2, wound on a6P slot frame, in a sequence of 12 slots whereof said full pitch coilsextend from slot 2 to slot 8, the further coils of the first windingcomponent extend from slot 3 to slot 7, the coils of the second windingcomponent extend from slot 5 to slot 10, and slot 11 to slot 4, andslots 6 and 12 are unoccupied.
 4. An electric motor as claimed in claim2, wound on a 6P slot frame, in a sequence of twelve slots whereof saidfull pitch coils extend from slot 2 to slot 8, the further coils of thefirst winding component extend from slot 3 to slot 7, the coils of thesecond winding component extend from slot 6 to slot 11, and slot 12 toslot 5, and slots 4 and 10 are unoccupied.
 5. An electric motor asclaimed in claim 4, in the frame of which every 6th slot is not punched.6. An electric motor as claimed in claim 1, in which the first andsecond winding components are electrically spaced at an angle of 90 +or - 15*.
 7. An electric motor as claimed in claim 2, in which the fullpitch coils are wound with one-half the number of turns of each of thecoils of less than full pitch.